1. Field of the Invention
This invention relates to microelectromechanical devices, and more particularly, to a microelectromechanical device including a switch configured for active opening by application of a mechanical force.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality devices with circuits formed using integrated circuit (IC) technology. For example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance as compared to transistor switches formed from conventional IC technology. However, a persistent problem with implementation of MEMS switches has been the high voltage required (often about 40V or higher) to actuate the switches, as compared to typical IC operating voltages (about 5V or lower).
These relatively high actuation voltages of MEMS switches are caused at least in part by a tradeoff between the closing and opening effectiveness of a given switch design. For example, approaches to lowering the actuation voltage of switches have included reducing the stiffness of the switch beam and/or reducing the gap between the beam and the conductive pad. Unfortunately, these design changes typically result in making the switch more difficult to open. MEMS switch designs generally use an applied voltage to close the switch, and rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force or restoring force of the beam must typically counteract what is often called “stiction.” Stiction refers to various forces tending to make two surfaces stick together such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through metallic diffusion). In general, modifications to a switch which act to lower the closing voltage also tend to make the switch harder to open, such that efforts to form a switch with a lowered closing voltage can result in a switch which may not open reliably (or at all).
Electrostatically actuated MEMS switches, both cantilevers and straps, generally can not be forced to open simply by changing the polarity of voltage on the gate. This inability to open is also due to the nature of electrostatic attraction. Therefore, the switch must be designed so that the elastic energy stored in the deformed switch is sufficient to cause opening after the actuating voltage is removed. Push-pull operation can be achieved if a second electrode is provided above the beam, but this is complex. Another alternative is a double gate structure, which has active opening ability. Magnetically actuated structures can be made with active opening, but such structures generally require more complex material sets and higher actuation currents. Teeter-totter designs also have active opening properties because when one half of the beam is in contact the other half can be actuated to cause opening.
Teeter-totter designs, however, are not as simple as cantilevers due to the pivot structure, which must be robust. In addition, since the clearance at the closed side is smaller than that at the open side, opening requires a larger voltage. For instance, if a closed dimple-less teeter-totter has an average clearance over the gate of the closed side of 1 micron, the average clearance over the gate of the open side is 2 microns. The voltage applied to the closing side creates a certain contact force at the contact. The same voltage applied to the opening side will create only ¼ the opening force. If the switch contact tends to stick with a sticking force comparable to the actuating force, twice the voltage must be applied to the opening side to break contact. However, it may be desirable to have several times greater opening force than closing force, implying even larger voltage differences. Currently, these problems are addressed by making the gate and beam on the opening side larger than that on the closing side. Other solutions are to add dimples to the contact, which limit the angular travel of the switch and make the opening side gap at the gate similar to that on the closing side.
It would therefore be desirable to develop a MEMS device which relaxes the constraints imposed by the above-described tradeoff between opening and closing effectiveness.